Article pubs.acs.org/accounts
Self-Assembled Three-Dimensional Graphene Macrostructures: Synthesis and Applications in Supercapacitors Yuxi Xu,§ Gaoquan Shi,*,# and Xiangfeng Duan*,§ §
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States Department of Chemistry, Tsinghua University, Beijing 100084, China
#
CONSPECTUS: Graphene and its derivatives are versatile building blocks for bottom-up assembly of advanced functional materials. In particular, with exceptionally large specific surface area, excellent electrical conductivity, and superior chemical/electrochemical stability, graphene represents the ideal material for various electrochemical energy storage devices including supercapacitors. However, due to the strong π−π interaction between graphene sheets, the graphene flakes tend to restack to form graphite-like powders when they are processed into practical electrode materials, which can greatly reduce the specific surface area and lead to inefficient utilization of the graphene layers for electrochemical energy storage. The self-assembly of two-dimensional graphene sheets into three-dimensional (3D) framework structures can largely retain the unique properties of individual graphene sheets and has recently garnered intense interest for fundamental investigations and potential applications in diverse technologies. In this Account, we review the recent advances in preparing 3D graphene macrostructures and exploring them as a unique platform for supercapacitor applications. We first describe the synthetic strategies, in which reduction of a graphene oxide dispersion above a certain critical concentration can induce the reduced graphene oxide sheets to cross-link with each other via partial π−π stacking interactions to form a 3D interconnected porous macrostructure. Multiple reduction strategies, including hydrothermal/solvothermal reduction, chemical reduction, and electrochemical reduction, have been developed for the preparation of 3D graphene macrostructures. The versatile synthetic strategies allow for easy incorporation of heteroatoms, carbon nanomaterials, functional polymers, and inorganic nanostructures into the macrostructures to yield diverse composites with tailored structures and properties. We then summarize the applications of the 3D graphene macrostructures for high-performance supercapacitors. With a unique framework structure in which the graphene sheets are interlocked in 3D space to prevent their restacking, the graphene macrostructures feature very high specific surface areas, rapid electron and ion transport, and superior mechanical strength. They can thus be directly used as supercapacitor electrodes with excellent specific capacitances, rate capabilities, and cycling stabilities. We finally discuss the current challenges and future opportunities in this research field. By regarding the graphene as both a single-atom-thick carbon sheet and a conjugated macromolecule, our work opens a new avenue to bottom-up self-assembly of graphene macromolecule sheets into functional 3D graphene macrostructures with remarkable electrochemical performances. We hope that this Account will promote further efforts toward fundamental investigation of graphene self-assembly and the development of advanced 3D graphene materials for their real-world applications in electrochemical energy storage devices and beyond.
1. INTRODUCTION
organizing building blocks into complex architectures via hydrogen bonding, π−π stacking interactions, electrostatic forces, etc. As the precursor of RGO, GO prepared by chemical oxidation of graphite can be well dispersed in many polar solvents, especially in water, at high concentrations and can be easily converted into RGO with largely restored conjugation by diverse reduction methods.8 Therefore, GO and RGO sheets can be regarded not only as carbon nanomaterials but also as two-dimensional (2D) conjugated macromolecules with extremely large molar masses based on their micrometer- or submicrometer-scale lateral sizes and naturally become versatile building blocks for the self-assembly of advanced materials with designed superstructures and functions.9−19
Graphene, a monolayer of graphite, has shown a wide range of promising applications, including electronic devices, energy storage and conversion, and polymer composites, since its first isolation in 2004.1−6 Several methods such as mechanical exfoliation, chemical vapor deposition, and reduction of graphene oxide (GO) have been widely explored to produce graphene for fundamental and applied research.1−8 Among these, reduction of GO has attracted particular interest in the chemistry and materials communities because of its capability for low-cost and high-throughput production of reduced graphene oxide (RGO).8 It is recognized that controlled preparation of graphene materials with well-defined hierarchical structures will pave the way for many practical applications that need bulk graphene materials. Self-assembly can be expected to be an effective technique for this purpose, which is capable of © XXXX American Chemical Society
Received: March 9, 2015
A
DOI: 10.1021/acs.accounts.5b00117 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 1. (a) Photographs of a 2 mg/mL homogeneous GO aqueous dispersion before and after hydrothermal reduction at 180 °C for 12 h. (b) Photographs of a strong graphene hydrogel allowing easy handling and weight support. (c−e) Scanning electron microscope (SEM) images with different magnifications of the graphene hydrogel’s interior microstructures. (f) Room temperature I−V curve of the graphene hydrogel exhibiting Ohmic characteristics. Inset shows the two-probe method for the conductivity measurements. Reproduced with permission from ref 28. Copyright 2010 American Chemical Society.
2. SYNTHETIC STRATEGIES
Electrochemical energy storage devices are of great importance for mobile electronics, electrical vehicles, and renewable energy harvesting, conversion, and storage.20 Graphene has attracted considerable interest in this regard because of its multiple appealing features, including high specific surface area, excellent electrical conductivity, and extraordinary chemical/electrochemical stability and mechanical flexibility.21 However, the strong van der Waals and π−π stacking interactions between graphene sheets make them readily aggregate to form graphite-like powders with compact layered structures when processed into bulk electrode materials, leading to a great loss of specific surface area and thus inefficient utilization of graphene layers for electrochemical energy storage. Moreover, electrochemically inactive polymer binders and/or conductive additives are usually needed to combine these graphite-like graphene powders into practical electrodes, which not only complicates the electrode preparation process but also introduces additional loss in the electrochemical performance. Self-assembly of nanoscale graphene into monolithic macroscopic materials with three-dimensional (3D) porous networks can largely translate the properties of individual graphene into the resulting macrostructures and simplify the processing of graphene materials. It has therefore garnered intense interest in the past few years.22−27 In this Account, we will present our recent advances in developing synthetic strategies to prepare 3D self-assembled graphene macrostructures starting with GO and exploring these materials for the creation of supercapacitors with unprecedented performance.
2.1. Hydrothermal or Solvothermal Reduction
We have synthesized the first self-assembled 3D graphene macrostructure, that is, graphene hydrogels, in 2010. The graphene hydrogel was produced by a convenient one-step hydrothermal reduction of a highly concentrated GO aqueous dispersion (Figure 1).28 A typical graphene hydrogel consists of a highly interconnected 3D graphene network (∼2 wt %) filled with water (∼98 wt %). The pore sizes in the 3D graphene range from submicrometer to several micrometers, and pore walls consist of thin layers of stacked graphene sheets. We proposed the self-assembly mechanism as follows: before reduction, the separated GO sheets were randomly and uniformly dispersed in water, due to their strong hydrophilicity and electrostatic repulsion effect. When GO was hydrothermally reduced, the oxygenated functionalities decreased significantly and the π-conjugation was largely restored. The π−π stacking interaction combined with hydrophobic effect promoted flexible RGO sheets to partially overlap and interlock with each other in 3D space to generate enough physical crosslink sites for the formation of a porous framework with water entrapped inside. Due to the unique self-assembled structure and superior graphene building block, the graphene hydrogel showed a high electrical conductivity of 0.5 S/m, a hierarchical meso- and macroporosity with a large specific surface area of 960 m2/g, and excellent mechanical strength with a storage modulus of 450−490 kPa, which is about 1−3 orders of magnitude higher B
DOI: 10.1021/acs.accounts.5b00117 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 2. (a) Photograph of an aqueous mixture of GO (2 mg/mL) and sodium ascorbate before (left) and after (right) chemical reduction at 90 °C for 1.5 h. Reproduced with permission from ref 46. Copyright 2011 Elsevier Ltd. (b) Schematic illustration of synthesis of anthraquinone-grafted graphene hydrogel. (c) SEM image of anthraquinone-modified graphene hydrogel and its photograph shown in the inset. Reproduced with permission from ref 49. Copyright 2011 Royal Society of Chemistry. (d) Schematic illustration of hydroquinone functionalized graphene hydrogel and its photograph shown in the inset. Reproduced with permission from ref 50. Copyright 2013 Wiley-VCH.
phene/polymers,37,38 graphene/metal nanoparticles,39−41 and graphene/metal oxide and sulfide42−44 composite hydrogels/ aerogels using the hydrothermal or solvothermal reduction strategies, which greatly expand the diversity and functions of 3D self-assembled graphene macrostructures. More recently, we have prepared a new 3D graphene macrostructure built from holey graphene sheets through a one-step hydrothermal process with simultaneous etching of nanopores within the graphene plane and self-assembly of graphene into a 3D network, which exhibited a highly continuous porous network of open channels with a ultrahigh accessible surface area of 1560 m2/g.45
than those of conventional self-assembled hydrogels. This initial study provides a new fundamental understanding of the selfassembly mechanism of graphene as a 2D macromolecule building block and inspires abundant subsequent studies on 3D graphene macrostructures. Soon after this work, we further used solvothermal reduction to prepare self-assembled graphene organogel starting with a GO dispersion in propylene carbonate.29 The graphene organogel showed similar porous network and mechanical strength and even a higher electrical conductivity of 2 S/m. These graphene hydrogels/organogels can be easily converted into graphene aerogels by using a freezing-drying or supercritical drying process. The hydrothermal and solvothermal reduction strategies are the most direct route to produce graphene gels without introducing any other chemicals or further purification treatment. Additionally, these processes are compatible with synthesis of many functional materials, which allows for convenient incorporation of a variety of secondary components into the 3D graphene framework to tailor the structures and combine the functions of multiple components. For example, we have demonstrated a one-step hydrothermal approach for the synthesis of graphene/Ni(OH)2 composite hydrogels by using an aqueous mixture of GO and nickel nitrate as the starting materials.30 The hydrothermal process not only induced the 3D self-assembly of graphene but also simultaneously promoted uniform in situ growth of ultrathin crystalline Ni(OH)2 nanoplates on the graphene framework. Utilizing the amphiphilic feature of GO, we have dispersed activated carbon particles into propylene carbonate and prepared graphene/activated carbon composite organogels by solvothermal reduction of the mixture dispersion.31 The resulting organogels consisted of a 3D graphene porous network with encapsulated carbon particles. Other research groups have also synthesized heteroatom-doped graphene hydrogels/aerogels,32−34 graphene/carbon nanotubes,35,36 gra-
2.2. Chemical Reduction
Although the hydrothermal or solvothermal strategies have their own features, the reactions require high temperature and high pressure, which is time-consuming and energy-consuming. Therefore, a milder method is needed for producing 3D selfassembled graphene macrostructures in large scale. Recognizing that the self-assembly of individual graphene sheets into a 3D network is mainly induced by the increased π−π stacking interaction between RGO during the hydrothermal reduction process, it is reasonable to believe that a chemical reduction process could also initiate the self-assembly of the 3D graphene network. Indeed, we have successfully synthesized graphene hydrogels by using sodium ascorbate as a reducing agent in the GO aqueous dispersion (Figure 2a).46 In contrast to the high temperature (180 °C) and long time (12 h) used in hydrothermal reduction, the chemical reduction process can be completed within less than 2 h below 100 °C. Moreover, the reaction is not limited to the autoclaves used in hydrothermal reduction and the shape of the graphene hydrogels can be easily varied by changing the type of reactor. The graphene hydrogels prepared by chemical reduction showed similar porous structure and mechanical properties. It is notable that their electrical conductivity (1−2 S/m) is higher C
DOI: 10.1021/acs.accounts.5b00117 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 3. Photograph (a) and cross-sectional SEM image (b) of an electrodeposited graphene hydrogel thin film electrode. (c, d) Top-view SEM images with low (c) and high (d) magnifications. Reproduced with permission from ref 54. Copyright 2012 Nature Publishing Group. (e) Schematic illustration of the electrochemical reduction method for producing graphene hydrogel film on electrode. Reproduced with permission from ref 55. Copyright 2012 Royal Society of Chemistry.
surface. The thickness of graphene hydrogel film on the electrode was found to increase with the electrodeposition time and could reach several millimeters after long deposition time. The electrochemical reduction of GO can be carried out on different types of electrodes including graphene paper, nickel foam, and metal foil or fiber, which thus is a universal strategy for direct preparation of graphene hydrogels on electrodes.55,56 The porous graphene hydrogels on electrodes are conductive and highly accessible for electrolyte and thus can be used as a new electrode for further electrodeposition of secondary functional components such as conducting polymers, metal nanoparticles, and metal oxide onto the graphene porous network to make composite hydrogels.55 For example, a graphene/polyaniline composite hydrogel can be readily prepared by a one-step cyclic voltammetry co-deposition process at a potential range of −1.2 to 0.8 V in a mixture dispersion of GO and aniline, during which GO sheets were electrochemically reduced to form a graphene hydrogel layer on the substrate electrode at negative potential, and aniline monomers were polymerized and homogeneously coated on the surfaces of RGO sheets at positive potential.57
than that of materials produced by hydrothermal reduction, likely due to a more complete reduction and restoration of πconjugation. Nearly concurrently, the chemical reduction approach has been reported by several groups to be an effective strategy to realize the 3D self-assembly of graphene, although with different reducing reagents such as ascorbic acid, NaHSO3, Na2S, and HI.47,48 The mild reaction conditions of the chemical reduction process can allow for preparation of graphene hydrogels modified with redox active anthraquinone through a two-step procedure in which anthraquinone covalently grafted GO was first synthesized and then chemically reduced by sodium ascorbate (Figure 2b,c).49 After chemical reduction, only a small amount of anthraquinone was removed, and the chemically modified graphene hydrogels contained 16 wt % anthraquinone. To simplify the preparation procedure, we have further synthesized noncovalently functionalized graphene hydrogels through a convenient one-step chemical reduction of GO using hydroquinones as both the reducing and functionalizing molecules (Figure 2d).50 The functionalized graphene hydrogels showed a high specific surface area of 1380 m2/g, which could accommodate a large content of redox active hydroquinone molecules, up to 17 wt %, on the graphene surface via π−π interactions. Similar to hydrothermal reduction, the chemical reduction strategy has also been widely used to prepare a variety of graphene composite hydrogels/aerogels with secondary components incorporated either before or during the self-assembly process.51−53
3. APPLICATIONS IN SUPERCAPACITORS Supercapacitors, also known as electrochemical capacitors, represent an important kind of electrochemical energy storage device with high power density, long cycle life, and high rate capability.58 The commercial supercapacitors are mainly based on activated carbon with unsatisfactory specific capacitances (
Self-Assembled Three-Dimensional Graphene Macrostructures: Synthesis and Applications in Supercapacitors Yuxi Xu,§ Gaoquan Shi,*,# and Xiangfeng Duan*,§ §
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States Department of Chemistry, Tsinghua University, Beijing 100084, China
#
CONSPECTUS: Graphene and its derivatives are versatile building blocks for bottom-up assembly of advanced functional materials. In particular, with exceptionally large specific surface area, excellent electrical conductivity, and superior chemical/electrochemical stability, graphene represents the ideal material for various electrochemical energy storage devices including supercapacitors. However, due to the strong π−π interaction between graphene sheets, the graphene flakes tend to restack to form graphite-like powders when they are processed into practical electrode materials, which can greatly reduce the specific surface area and lead to inefficient utilization of the graphene layers for electrochemical energy storage. The self-assembly of two-dimensional graphene sheets into three-dimensional (3D) framework structures can largely retain the unique properties of individual graphene sheets and has recently garnered intense interest for fundamental investigations and potential applications in diverse technologies. In this Account, we review the recent advances in preparing 3D graphene macrostructures and exploring them as a unique platform for supercapacitor applications. We first describe the synthetic strategies, in which reduction of a graphene oxide dispersion above a certain critical concentration can induce the reduced graphene oxide sheets to cross-link with each other via partial π−π stacking interactions to form a 3D interconnected porous macrostructure. Multiple reduction strategies, including hydrothermal/solvothermal reduction, chemical reduction, and electrochemical reduction, have been developed for the preparation of 3D graphene macrostructures. The versatile synthetic strategies allow for easy incorporation of heteroatoms, carbon nanomaterials, functional polymers, and inorganic nanostructures into the macrostructures to yield diverse composites with tailored structures and properties. We then summarize the applications of the 3D graphene macrostructures for high-performance supercapacitors. With a unique framework structure in which the graphene sheets are interlocked in 3D space to prevent their restacking, the graphene macrostructures feature very high specific surface areas, rapid electron and ion transport, and superior mechanical strength. They can thus be directly used as supercapacitor electrodes with excellent specific capacitances, rate capabilities, and cycling stabilities. We finally discuss the current challenges and future opportunities in this research field. By regarding the graphene as both a single-atom-thick carbon sheet and a conjugated macromolecule, our work opens a new avenue to bottom-up self-assembly of graphene macromolecule sheets into functional 3D graphene macrostructures with remarkable electrochemical performances. We hope that this Account will promote further efforts toward fundamental investigation of graphene self-assembly and the development of advanced 3D graphene materials for their real-world applications in electrochemical energy storage devices and beyond.
1. INTRODUCTION
organizing building blocks into complex architectures via hydrogen bonding, π−π stacking interactions, electrostatic forces, etc. As the precursor of RGO, GO prepared by chemical oxidation of graphite can be well dispersed in many polar solvents, especially in water, at high concentrations and can be easily converted into RGO with largely restored conjugation by diverse reduction methods.8 Therefore, GO and RGO sheets can be regarded not only as carbon nanomaterials but also as two-dimensional (2D) conjugated macromolecules with extremely large molar masses based on their micrometer- or submicrometer-scale lateral sizes and naturally become versatile building blocks for the self-assembly of advanced materials with designed superstructures and functions.9−19
Graphene, a monolayer of graphite, has shown a wide range of promising applications, including electronic devices, energy storage and conversion, and polymer composites, since its first isolation in 2004.1−6 Several methods such as mechanical exfoliation, chemical vapor deposition, and reduction of graphene oxide (GO) have been widely explored to produce graphene for fundamental and applied research.1−8 Among these, reduction of GO has attracted particular interest in the chemistry and materials communities because of its capability for low-cost and high-throughput production of reduced graphene oxide (RGO).8 It is recognized that controlled preparation of graphene materials with well-defined hierarchical structures will pave the way for many practical applications that need bulk graphene materials. Self-assembly can be expected to be an effective technique for this purpose, which is capable of © XXXX American Chemical Society
Received: March 9, 2015
A
DOI: 10.1021/acs.accounts.5b00117 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 1. (a) Photographs of a 2 mg/mL homogeneous GO aqueous dispersion before and after hydrothermal reduction at 180 °C for 12 h. (b) Photographs of a strong graphene hydrogel allowing easy handling and weight support. (c−e) Scanning electron microscope (SEM) images with different magnifications of the graphene hydrogel’s interior microstructures. (f) Room temperature I−V curve of the graphene hydrogel exhibiting Ohmic characteristics. Inset shows the two-probe method for the conductivity measurements. Reproduced with permission from ref 28. Copyright 2010 American Chemical Society.
2. SYNTHETIC STRATEGIES
Electrochemical energy storage devices are of great importance for mobile electronics, electrical vehicles, and renewable energy harvesting, conversion, and storage.20 Graphene has attracted considerable interest in this regard because of its multiple appealing features, including high specific surface area, excellent electrical conductivity, and extraordinary chemical/electrochemical stability and mechanical flexibility.21 However, the strong van der Waals and π−π stacking interactions between graphene sheets make them readily aggregate to form graphite-like powders with compact layered structures when processed into bulk electrode materials, leading to a great loss of specific surface area and thus inefficient utilization of graphene layers for electrochemical energy storage. Moreover, electrochemically inactive polymer binders and/or conductive additives are usually needed to combine these graphite-like graphene powders into practical electrodes, which not only complicates the electrode preparation process but also introduces additional loss in the electrochemical performance. Self-assembly of nanoscale graphene into monolithic macroscopic materials with three-dimensional (3D) porous networks can largely translate the properties of individual graphene into the resulting macrostructures and simplify the processing of graphene materials. It has therefore garnered intense interest in the past few years.22−27 In this Account, we will present our recent advances in developing synthetic strategies to prepare 3D self-assembled graphene macrostructures starting with GO and exploring these materials for the creation of supercapacitors with unprecedented performance.
2.1. Hydrothermal or Solvothermal Reduction
We have synthesized the first self-assembled 3D graphene macrostructure, that is, graphene hydrogels, in 2010. The graphene hydrogel was produced by a convenient one-step hydrothermal reduction of a highly concentrated GO aqueous dispersion (Figure 1).28 A typical graphene hydrogel consists of a highly interconnected 3D graphene network (∼2 wt %) filled with water (∼98 wt %). The pore sizes in the 3D graphene range from submicrometer to several micrometers, and pore walls consist of thin layers of stacked graphene sheets. We proposed the self-assembly mechanism as follows: before reduction, the separated GO sheets were randomly and uniformly dispersed in water, due to their strong hydrophilicity and electrostatic repulsion effect. When GO was hydrothermally reduced, the oxygenated functionalities decreased significantly and the π-conjugation was largely restored. The π−π stacking interaction combined with hydrophobic effect promoted flexible RGO sheets to partially overlap and interlock with each other in 3D space to generate enough physical crosslink sites for the formation of a porous framework with water entrapped inside. Due to the unique self-assembled structure and superior graphene building block, the graphene hydrogel showed a high electrical conductivity of 0.5 S/m, a hierarchical meso- and macroporosity with a large specific surface area of 960 m2/g, and excellent mechanical strength with a storage modulus of 450−490 kPa, which is about 1−3 orders of magnitude higher B
DOI: 10.1021/acs.accounts.5b00117 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 2. (a) Photograph of an aqueous mixture of GO (2 mg/mL) and sodium ascorbate before (left) and after (right) chemical reduction at 90 °C for 1.5 h. Reproduced with permission from ref 46. Copyright 2011 Elsevier Ltd. (b) Schematic illustration of synthesis of anthraquinone-grafted graphene hydrogel. (c) SEM image of anthraquinone-modified graphene hydrogel and its photograph shown in the inset. Reproduced with permission from ref 49. Copyright 2011 Royal Society of Chemistry. (d) Schematic illustration of hydroquinone functionalized graphene hydrogel and its photograph shown in the inset. Reproduced with permission from ref 50. Copyright 2013 Wiley-VCH.
phene/polymers,37,38 graphene/metal nanoparticles,39−41 and graphene/metal oxide and sulfide42−44 composite hydrogels/ aerogels using the hydrothermal or solvothermal reduction strategies, which greatly expand the diversity and functions of 3D self-assembled graphene macrostructures. More recently, we have prepared a new 3D graphene macrostructure built from holey graphene sheets through a one-step hydrothermal process with simultaneous etching of nanopores within the graphene plane and self-assembly of graphene into a 3D network, which exhibited a highly continuous porous network of open channels with a ultrahigh accessible surface area of 1560 m2/g.45
than those of conventional self-assembled hydrogels. This initial study provides a new fundamental understanding of the selfassembly mechanism of graphene as a 2D macromolecule building block and inspires abundant subsequent studies on 3D graphene macrostructures. Soon after this work, we further used solvothermal reduction to prepare self-assembled graphene organogel starting with a GO dispersion in propylene carbonate.29 The graphene organogel showed similar porous network and mechanical strength and even a higher electrical conductivity of 2 S/m. These graphene hydrogels/organogels can be easily converted into graphene aerogels by using a freezing-drying or supercritical drying process. The hydrothermal and solvothermal reduction strategies are the most direct route to produce graphene gels without introducing any other chemicals or further purification treatment. Additionally, these processes are compatible with synthesis of many functional materials, which allows for convenient incorporation of a variety of secondary components into the 3D graphene framework to tailor the structures and combine the functions of multiple components. For example, we have demonstrated a one-step hydrothermal approach for the synthesis of graphene/Ni(OH)2 composite hydrogels by using an aqueous mixture of GO and nickel nitrate as the starting materials.30 The hydrothermal process not only induced the 3D self-assembly of graphene but also simultaneously promoted uniform in situ growth of ultrathin crystalline Ni(OH)2 nanoplates on the graphene framework. Utilizing the amphiphilic feature of GO, we have dispersed activated carbon particles into propylene carbonate and prepared graphene/activated carbon composite organogels by solvothermal reduction of the mixture dispersion.31 The resulting organogels consisted of a 3D graphene porous network with encapsulated carbon particles. Other research groups have also synthesized heteroatom-doped graphene hydrogels/aerogels,32−34 graphene/carbon nanotubes,35,36 gra-
2.2. Chemical Reduction
Although the hydrothermal or solvothermal strategies have their own features, the reactions require high temperature and high pressure, which is time-consuming and energy-consuming. Therefore, a milder method is needed for producing 3D selfassembled graphene macrostructures in large scale. Recognizing that the self-assembly of individual graphene sheets into a 3D network is mainly induced by the increased π−π stacking interaction between RGO during the hydrothermal reduction process, it is reasonable to believe that a chemical reduction process could also initiate the self-assembly of the 3D graphene network. Indeed, we have successfully synthesized graphene hydrogels by using sodium ascorbate as a reducing agent in the GO aqueous dispersion (Figure 2a).46 In contrast to the high temperature (180 °C) and long time (12 h) used in hydrothermal reduction, the chemical reduction process can be completed within less than 2 h below 100 °C. Moreover, the reaction is not limited to the autoclaves used in hydrothermal reduction and the shape of the graphene hydrogels can be easily varied by changing the type of reactor. The graphene hydrogels prepared by chemical reduction showed similar porous structure and mechanical properties. It is notable that their electrical conductivity (1−2 S/m) is higher C
DOI: 10.1021/acs.accounts.5b00117 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 3. Photograph (a) and cross-sectional SEM image (b) of an electrodeposited graphene hydrogel thin film electrode. (c, d) Top-view SEM images with low (c) and high (d) magnifications. Reproduced with permission from ref 54. Copyright 2012 Nature Publishing Group. (e) Schematic illustration of the electrochemical reduction method for producing graphene hydrogel film on electrode. Reproduced with permission from ref 55. Copyright 2012 Royal Society of Chemistry.
surface. The thickness of graphene hydrogel film on the electrode was found to increase with the electrodeposition time and could reach several millimeters after long deposition time. The electrochemical reduction of GO can be carried out on different types of electrodes including graphene paper, nickel foam, and metal foil or fiber, which thus is a universal strategy for direct preparation of graphene hydrogels on electrodes.55,56 The porous graphene hydrogels on electrodes are conductive and highly accessible for electrolyte and thus can be used as a new electrode for further electrodeposition of secondary functional components such as conducting polymers, metal nanoparticles, and metal oxide onto the graphene porous network to make composite hydrogels.55 For example, a graphene/polyaniline composite hydrogel can be readily prepared by a one-step cyclic voltammetry co-deposition process at a potential range of −1.2 to 0.8 V in a mixture dispersion of GO and aniline, during which GO sheets were electrochemically reduced to form a graphene hydrogel layer on the substrate electrode at negative potential, and aniline monomers were polymerized and homogeneously coated on the surfaces of RGO sheets at positive potential.57
than that of materials produced by hydrothermal reduction, likely due to a more complete reduction and restoration of πconjugation. Nearly concurrently, the chemical reduction approach has been reported by several groups to be an effective strategy to realize the 3D self-assembly of graphene, although with different reducing reagents such as ascorbic acid, NaHSO3, Na2S, and HI.47,48 The mild reaction conditions of the chemical reduction process can allow for preparation of graphene hydrogels modified with redox active anthraquinone through a two-step procedure in which anthraquinone covalently grafted GO was first synthesized and then chemically reduced by sodium ascorbate (Figure 2b,c).49 After chemical reduction, only a small amount of anthraquinone was removed, and the chemically modified graphene hydrogels contained 16 wt % anthraquinone. To simplify the preparation procedure, we have further synthesized noncovalently functionalized graphene hydrogels through a convenient one-step chemical reduction of GO using hydroquinones as both the reducing and functionalizing molecules (Figure 2d).50 The functionalized graphene hydrogels showed a high specific surface area of 1380 m2/g, which could accommodate a large content of redox active hydroquinone molecules, up to 17 wt %, on the graphene surface via π−π interactions. Similar to hydrothermal reduction, the chemical reduction strategy has also been widely used to prepare a variety of graphene composite hydrogels/aerogels with secondary components incorporated either before or during the self-assembly process.51−53
3. APPLICATIONS IN SUPERCAPACITORS Supercapacitors, also known as electrochemical capacitors, represent an important kind of electrochemical energy storage device with high power density, long cycle life, and high rate capability.58 The commercial supercapacitors are mainly based on activated carbon with unsatisfactory specific capacitances (
